The present invention is directed to a digital system for measuring the density of a material in a rod-shaped package. More particularly, the present invention is directed to a digital cigarette rod density measurement system for obtaining a density profile of the tobacco contained in a cigarette rod.
Manufacturers of material packaged in cylindrical packages, for example, cigarette rods containing tobacco, are interested in ascertaining whether the manufacturing process is properly manufacturing cigarettes to the design specifications. One such design specification is the percentage of tobacco and air contained within a cigarette rod.
Cigarettes are typically manufactured such that a pre-determined amount of tobacco is contained within each rod. In order to determine whether the manufacturing process is operating properly, as well as to provide a tool for developing cigarette rod products which utilize more air and less tobacco in the cigarette rod, it is useful to be able to determine the weight or mass of tobacco contained within a cigarette rod.
Prior art devices for obtaining a rod density profile of a cigarette are known. One such device, however, only measures the rod density in an indirect manner using near near infrared radiation (NNIR). NNIR technology, while useful in some chemical determinations, has limitations when applied to measure the mass of tobacco in a cigarette. For example, such devices must be calibrated with dry samples in order to obtain a correlation for dry cigarette rod weight. Wet tobacco weight must be estimated after the moisture content of the tobacco is obtained by utilizing an additional method. Another significant limitation of NNIR technology is that NNIR radiation does not penetrate optically opaque material such as aluminum foil. This limitation presents a serious difficulty for known prior art devices when used with cigarette rods which incorporate a laminated aluminum foil wrap or other optically opaque materials.
The present invention, utilizes beta particles, which react directly to only the total mass of the material in their path. Such beta particles do penetrate aluminum foil. Thus, the present invention is able to account for the aluminum foil by using a weight offset. Utilizing the present invention, the greater the material present in front of the beta particles, the fewer particles that are transmitted through to a sensor on the other side of the material or cigarette rod. The remainder of the beta particles are scattered back by the material of the cigarette rod or are absorbed by that material.
The overall absorption of the beta particles has been found to follow an exponential law which can be expressed in the following equation: EQU I=I.sub.o e.sup.(-1.0*u*x)
where I.sub.o is the number of particles detected without the material, I is the number of particles detected after passing through the material, u is the mass absorption coefficient of the material in cm.sup.2 /g and x is the density thickness of the material expressed in g/cm.sup.2. Using this equation, the cigarette density measurement system of the present invention can determine the mass of a slice of a cigarette by measuring the particle count both with and without the cigarette slice in the path of the emitted beta particles. The measurement using the present invention is robust and does not suffer from the blend sensitivity exhibited by prior art devices.
Another shortcoming of the known devices used to obtain rod profile measurements is that such devices typically utilize analog detectors, even in instances where a beta gauge is utilized. A digital beta detector is significantly more efficient than the analog ion chamber utilized with such prior art devices and therefore a small source of radiation is utilized. Also, the use of an analog ion chamber requires complex analog circuitry with temperature compensation in order to process the output signal. The cigarette density measurement system of the present invention, on the other hand, produces a digital output which requires only a counter for accumulating the number of beta particles to obtain a reading, thus eliminating the need for temperature compensation in order to acquire accurate data. Lastly, the known prior art devices for obtaining cigarette rod profile measurements are much more costly to produce than the present invention.